9 ROM, EPROM, & EEPROM TECHNOLOGY Overview Read only memory devices are a special case of memory where, in normal system oper- ation, the memory is read but not changed. Read only memories are non-volatile, that is, stored information is retained when the power is removed. The main read only memory devices are listed below: ROM (Mask Programmable ROM —also called “MROMs”) EPROM (UV Erasable and Electrically Programmable ROM) OTP (One Time Programmable EPROM) EEPROM (Electrically Erasable and Programmable ROM) Flash Memory - These devices are covered in Section 10. How the Device Works The read only memory cell usu- Column ally consists of a single transistor (ROM and EPROM cells consist of one transistor, EEPROM cells consists of two transistors). The gate threshold voltage of the Row transistor determines whether it is a “1” or “0”. During the read Cell cycle, a voltage is placed on the Selected gate of the cell. Depending on the programmed threshold volt- Sense Amplifier age, the transistor will or will not Current Detector drive a current. The sense ampli- To Output Buffer fier will transform this current, or lack of current, into a “1” or Source: ICE, "Memory 1996" 19956 “0”. Figure 9-1 shows how a Figure 9-1. Read Only Memory Schematic read only memory works. INTEGRATED CIRCUIT ENGINEERING CORPORATION 9-1 ROM, EPROM, & EEPROM Technology Mask Programmable ROMs Mask programmable ROMs (ROMs) are the least expensive type of solid state memory. They are primarily used for storing video game software and fixed data storage for elec- tronic equipment, such as fonts for laser printers, dictionary data in word processors, and sound data in electronic musical instruments. ROM programming is performed during IC fabrication. Several process alternatives can be used to program a ROM. These are: • Metal contact to connect (or not to connect) a transistor to the bit line. • Channel implant to create either an enhancement-mode transistor or a depletion- mode transistor. • Thin or thick gate oxide, which creates either a standard transistor or a high threshold transistor, respectively. The choice of these is a trade-off between process complexity, chip size, and manufactur- ing cycle time. A ROM programmed at the metal contact level will have the shortest manufacturing cycle time, as metallization is one of the last process steps. However, the size of the cell will be larger. Figure 9-2 shows a ROM array programmed by channel implant. The transistor cell will have either a normal threshold (enhancement-mode device) or a very high threshold (higher than Vcc to assure the transistor will always be off). The cell array architecture is NOR. The different types of ROM architectures (NOR, NAND, ...) are detailed in the flash memory section. Figure 9-3 shows an array of storage cells (NAND architecture) that consists of single transistors illustrated as devices 1 through 10 and 11 through 20 that is programmed with either a normal threshold (enhancement-mode device) or a negative threshold (deple- tion-mode device). The devices are read sequentially within the small groups. Two groups are shown in the drawing. In 1995, a handful of manufacturers offered 64Mbit ROM devices. Power supplies ranged between 2.7V and 5.5V with access times in the range of 150ns. Hitachi announced a 3.3V 16Mbit ROM with a x32bit or x16bit organization and a burst mode (40ns access time). Random access time on the device was 120ns. 9-2 INTEGRATED CIRCUIT ENGINEERING CORPORATION ROM, EPROM, & EEPROM Technology Ground Diffusion Selective Implant To Raise VT ROW 1 (Polysilicon) ROW 1 T3 T4 Drain Contacts: T4 T3 Shared By 2 Bits Drain Diffusion ROW 2 ROW 2 (Polysilicon) T1 T2 T1 T2 Ground Metal Columns (Not Drawn) Source: ICE, "Memory 1996" 20845 Figure 9-2. ROM Programmed by Channel Implant 1 WORD 1/11 11 2 WORD 2/12 12 3 WORD 3/13 13 4 WORD 4/14 14 5 WORD 5/15 15 6 WORD 6/16 16 7 WORD 7/17 17 8 WORD 8/18 18 9 CONTROL LINE 19 10 SELECT LINE 20 BIT LINE Source: ICE, "Memory 1996" 19050 Figure 9-3. Memory Cell Schematic INTEGRATED CIRCUIT ENGINEERING CORPORATION 9-3 ROM, EPROM, & EEPROM Technology Sharp integrated on a single chip a ROM and a Pseudo SRAM (PSRAM). Sharp called its 8Mbit device a ROM/RAM. The chip, configured 512Kbit x 16, contains 8Mbit of RAM and 2Kbit of ROM. This device: • Allows RAM and ROM to be accessed at the same system speed to simplify address management and ease software development. • Saves board space by combining functions that otherwise require multiple chips. The ROM/RAM device has an access time of 80ns (170ns cycle time) and the power sup- ply is 3V. The proposed application for DOS systems is shown in Figure 9-4. FFFFF FFFFF ROM BIOS F0000 ROM DOS E0000 RAM Expansion I/O ROM ROM C0000 Possible to arrange Video RAM RAM ROM and RAM freely together on the A0000 ROM/RAM Area entire memory area. Application Area ROM 1Mbyte DOS System Memory Data Area RAM DOS Data 00000 00000 Interupt Vector 8M ROM/RAM Coexistent Memory ROM Data Mapping Source: Sharp 19957 Figure 9-4. Sharp ROM/RAM Configuration Proposal EPROM EPROM (UV Erasable Programmable Read Only Memory) is a special type of ROM that is programmed in finished form (after device packaging), usually by the end user or sys- tem manufacturer. 9-4 INTEGRATED CIRCUIT ENGINEERING CORPORATION ROM, EPROM, & EEPROM Technology The EPROM device is programmed by forcing an electrical charge on a small piece of polysilicon material (called a floating storage gate) located in the memory cell. When this charge is present on this gate, the cell is “programmed,” usually a logic “0,” and when it is not present, it is a logic “1.” Figure 9-5 shows the cell used in a typical EPROM. The floating gate is where the electrical charge is stored. First-Level Polysilicon +VG Second-Level (Floating) Polysilicon Gate Oxide Field Oxide – – – N+ P- Substrate Source: Intel 18474 Figure 9-5. Double-Poly Structure (EPROM/Flash Memory Cell) Prior to being programmed, an EPROM has to be erased. The EPROM is exposed to an ultraviolet light for approximately 20 minutes through a quartz window in its ceramic package. After erasure, new information can be programmed to the EPROM. After writ- ing the data to the EPROM, an opaque label is placed over the quartz window to prevent accidental erasure. Programming is accomplished through a phenomenon called hot electron injection. High voltages are applied to the select gate and drain connections of the cell transistor. The select gate of the transistor is pulsed “on” causing a large drain current to flow. The large bias voltage on the gate connection attracts electrons that penetrate the thin gate oxide and are stored on the floating gate. EPROM Floating Gate Transistor Characteristic Theory The following explanation is also true for EEPROM and flash devices. Figure 9-6 (a) and (b) shows the cross section of a conventional MOS transistor and a floating gate transistor, respectively. The upper gate in Figure 9-6 (b) is the control gate and the lower gate, completely isolated within the gate oxide, is the floating gate. CFG INTEGRATED CIRCUIT ENGINEERING CORPORATION 9-5 ROM, EPROM, & EEPROM Technology and CFS are the capacitances between the floating gate and the control gate and substrate, respectively. VG and VF are the voltages of the control gate and the floating gate, respec- tively. -QF is the charge in the floating gate. (As electrons have a negative charge, we added a negative sign). In an equilibrium state, the sum of the charges equals zero. Control G Gate Floating Control Gate Gate G S D S D CFG CFS N+ N+ N+ N+ (a) Conventional MOS (b) Floating-Gate MOS Source: ICE, "Memory 1996" 20846 Figure 9-6. Cross Section of a Conventional MOS Transistor and a Floating-Gate MOS Transistor (VG - VF) CFG + (0 - VF) CFS - QF = 0 CFG QF VF =VG - CFG + CFS CFG + CFS VTC is the threshold voltage of the conventional transistor, and VTCG is the threshold voltage of the floating gate transistor: CFG QF VTCG = VTC - CFG + CFS CFG + CFS QF VTCG = VTO - CG CFG Where VTO = VTC and CG = CFG + CFS CFG + CFS 9-6 INTEGRATED CIRCUIT ENGINEERING CORPORATION ROM, EPROM, & EEPROM Technology The threshold voltage of the floating gate transistor (VTCG) will be VTO (around 1V) plus a term depending on the charge trapped in the floating gate. If no electrons are in the floating gate, then VTCG = VTO (around 1V). If electrons have been trapped in the floating gate, then VTCG = VTO - QF/CG (around 8V for a 5V part). This voltage is process and design dependent. Figure 9-7 shows the threshold voltage shift of an EPROM cell. –QF VT = CG Erased State Programmed State (Logic "1") (Logic "0") Drain Current –Q Select Gate V V F T0 Sense T0 Voltage CG Threshold Source: ICE, "Memory 1996" 17548A Figure 9-7. Electrical Characteristics of an EPROM The programming (write cycle) of an EPROM takes several hundred milliseconds. Usually a byte — eight bits — is addressed with each write cycle. The read time is com- parable to that of fast ROMs and DRAMs (i.e., several tens of nanoseconds). In those applications where programs are stored in EPROMs, the CPU can run at normal speeds. Field programmability is the EPROM’s main advantage over the ROM. It allows the user to buy mass-produced devices and program each device for a specific need. This char- acteristic also makes the EPROM ideal for small-volume applications, as the devices are programmed in very small quantities.